This is a discussion on Crashed 04 WRX - ABS Failure? within the General Maintenance, Troubleshooting & Accidents. forums, part of the Tech & Modifying & General Repairs category; I got my 04 WRX 6 weeks ago. For the 5 weeks I drove it, the ABS would engage regularly ...

Crashed 04 WRX - ABS Failure?

I got my 04 WRX 6 weeks ago. For the 5 weeks I drove it, the ABS would engage regularly under heavy braking on uneven surfaces (as I would expect). Then a week ago I entered a turn at a speed well beyond what I should have, and crashed due to severe understeer. After the crash I realized my ABS never engaged. Do you have to bring your wheels to full lock for the ABS to engage? I thought the system just had to sense that the vehicle speed and wheel speed no longer matched perfectly. During the turn I got on the brakes hard to try to reduce understeer but I did not stand on them, so as to try to avoid full lock and/or oversteer. Was that what caused the ABS to not engage or might they have failed? If they should have engaged under the above conditions that may help me write off the crash as no-fault with the insurance company.

I got my 04 WRX 6 weeks ago. For the 5 weeks I drove it, the ABS would engage regularly under heavy braking on uneven surfaces (as I would expect). Then a week ago I entered a turn at a speed well beyond what I should have, and crashed due to severe understeer. After the crash I realized my ABS never engaged. Do you have to bring your wheels to full lock for the ABS to engage? I thought the system just had to sense that the vehicle speed and wheel speed no longer matched perfectly. During the turn I got on the brakes hard to try to reduce understeer but I did not stand on them, so as to try to avoid full lock and/or oversteer. Was that what caused the ABS to not engage or might they have failed? If they should have engaged under the above conditions that may help me write off the crash as no-fault with the insurance company.

0 The 5.3i type ABS used in the lmpreza has a hydraulic control unit, an ABS control module, a
valve relay and a motor relay integrated into a single unit (called “ABSCM & H/U”) for circuit simplicity
and reduced weight.
0 The ABS electrically controls the brake fluid pressure to each wheel to prevent the wheel from
locking during braking on slippery road surfaces, thereby enabling the driver to maintain the
directional control.
0 If the ABS becomes inoperative, a fail-safe system is activated to ensure same level of braking
performance as with a conventional brake system. In that case, the warning light comes on to indicate
that the ABS is malfunctioning.
0 The ABS is a 4-sensor, 4-channel system; the front wheel system is an independent control design*’,
while the rear wheel system is a select-low control design*2.
*I : A system which controls the front wheel brakes individually.
*2: A system which applies the same fluid pressure to both the rear wheels if either wheel starts
to lock. The pressure is determined based on the lower of the frictional coefficients of both wheels.

C: PRINCIPLE OF ABS CONTROL
When the brake pedal is depressed during driving, the wheel speed decreases and the vehicle
speed does as well. The decrease in the vehicle speed, however, is not always proportional to the
decrease in the wheel speed. The non-correspondence between the wheel speed and vehicle
speed is called “slip” and the magnitude of the slip is expressed by the “slip ratio” which is defined
as follows:
Slip ratio = Vehicle speed - Wheel speedNehicle speed x 100%
When the slip ratio is 0%, the vehicle speed corresponds exactly to the wheel speed; when it is
loo%, the wheels are completely locking (rotating at a zero speed) while the vehicle is moving.
The braking effectiveness is represented by the “coefficient of friction” between the tire and road
surface. The larger the Coefficient, the higher the braking effectiveness. The diagram below shows
the relationship between the coefficient of friction and the slip ratio for two different road surface
conditions (asphalt-paved road and icy road), assuming that the same tires are used for both the
conditions and the vehicles are moving forward. Although the braking effectiveness (coefficient
of friction) depends on the road surface condition as shown and also on the type of the tire, its peak
range generally corresponds to the 8 - 30% range of the slip ratio.
The ABS controls the fluid pressure to each wheel so that a coefficient of friction corresponding
to this dip ratio range is maintained.

To the second response, keep the useless comments coming. I admitted in my original post that I took a turn I had no business taking. I don't need a lesson in my degree of stupidity causing a car crash. I'm simply asking if the ABS SHOULD HAVE engaged for insurance purposes. The first response marginally helped with that, though I already understood the concept of how ABS works and the concepts behind slip.

I'm just looking for whether or not my ABS was supposed to engage. I don't need a reminder that I should tone down the driving. My destroyed suspension and axle told me that just fine.

To the second response, keep the useless comments coming. I admitted in my original post that I took a turn I had no business taking. I don't need a lesson in my degree of stupidity causing a car crash. I'm simply asking if the ABS SHOULD HAVE engaged for insurance purposes. The first response marginally helped with that, though I already understood the concept of how ABS works and the concepts behind slip.

I'm just looking for whether or not my ABS was supposed to engage. I don't need a reminder that I should tone down the driving. My destroyed suspension and axle told me that just fine.

No offense but if you knew all about the information above you wouldn't need to be asking here if your ABS should have engaged or not. You should have been able to deduce this yourself. I mean how are we supposed to know with out vehicle speed, g-force,wheel speed and all the other info ABS uses to calculate how to react. You would need all that info to answer that question plus the factory set thresholds.

I agree with Donkey. Also, it could be that you're assuming ABS is equivalent to what you are essentially describing as stability control, when in fact it is not -- stability control systems tend to use the ABS and throttle to achieve what you described as the result you expected, but ABS on its own engages when the wheels are locking up under hard braking. If the wheels are not on the threshold of locking, the ABS should stay out of things.

WRX Info Links, Courtesy TheJMan is a brute.... If you're cruel to him, he respects and fears you. If you're kind to him, he plucks your eyes out. Alexis ZorbasI lied. I cheated. I bribed men to cover the crimes of other men. I am an accessory to murder. But the most damning thing of all... I think I can live with it. And if I had to do it all over again - I would. Benjamin Sisko
DISCLAIMER: Opinions expressed are the author's alone and are inherently worthless.

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